1. Field of the Invention
The present invention relates to an electron gun for a color cathode-ray tube for enhancing convergence by efficiently focusing electron beams emitted from three cathodes of in-line alignment on a fluorescent screen and removing the flare of a beam spot which is produced around the fluorescent screen of the color cathode-ray tube in terms of the deflection magnetic field for self-convergence.
2. Description of the Prior Art
In general, several types of color cathode-ray tubes are known in the art. One of the color cathode-ray tubes is structured, as shown in FIG. 1, such that three electron beams Bs, Bc and Bs are emitted from an electron gun 2 contained in a neck portion 1 in the rear of a glass bulb and focused on a point of a shadow mask 3, and then combined with R.G.B. colors so as to reproduce desired images on a fluorescent screen 5 which is on the internal surface of a panel 4.
The electron gun 2 is of an in-line type for emitting three electron beams in parallel with the axis (A--A) of the color cathode-ray tube, and must have an electron beam focusing structure in order to focus the three parallel beams on one point of the fluorescent screen.
FIGS. 2 and 3 illustrate an electron gun 2 which is generally applied to a conventional color cathode-ray tube. As shown in FIGS. 2 and 3, the electron gun comprises three cathodes 7 each having a heater 6 therein, first and second grid electrodes 8 and 9, a first accelerating and focusing electrode 10 each of which has three electron beam passing holes 81, 82, 83, 91, 92, 93, 101, 102 and 103 being spaced from each other as much as a predetermined distance S and aligned in the same axial line, and second accelerating and focusing electrode 11 of which a central electron beam passing hole 112 is aligned in the same axial line as the electron beam passing holes 82, 92 and 102 of the first and second grid electrodes 8 and 9 and first accelerating and focusing electrode 10 and side electron passing holes 111 and 113 are aligned eccentrically to the electron beam passing holes 81, 83, 91, 93, 101 and 103 of the first and second grid electrodes 8 and 9 and first accelerating and focusing electrode as much as a predetermined distance .DELTA.S toward the outer side. In the above structure, the amount of eccentricity .DELTA.S is determined such that the diameters of the electron beam passing holes 111 and 113 of the second accelerating and focusing electrode 11 are larger than or the same as the diameters of the electron beam passing holes 101 and 103 of the first accelerating and focusing electrode 10, and the distance S' between the electron beam passing holes of the second accelerating and focusing electrode 11 is larger than the distance S between the electron beam passing holes of the first accelerating and focusing electrode 10.
FIG. 4 shows a conventional convergence structure in which the electron beam passing holes 101, 103 and 111, 113 of the first accelerating and focusing electrode 10 and the second accelerating and focusing electrode 11 are formed in with an amount of eccentricity .DELTA.S. When a voltage is applied from the outside of the electron gun 2, equipotential lines V1, V2 . . . , which are called a main electron lens are formed, for focusing the electron beams Bs, Bc and Bs at a space between the first and second accelerating and focusing electrodes 10 and 11 so that a plurality of electron beams which are emitted from the cathodes 7 can be focused on the fluorescent screen as a beam spot. The equipotential lines at the second accelerating and focusing electrode 11 are formed asymmetrically with respect to the electron beam path between the electron beam passing holes 101, 103, 111, and 113, by the eccentricity .DELTA.S.
Accordingly, the electron beam Bs which passes through the above path advances refractively toward the central beam Bc as much as a predetermined angle .theta.' by an equation of refraction and then focused on a point on the fluorescent screen 5.
Meanwhile, the main electron lens formed between the first accelerating and focusing electrode 10 and the second accelerating and focusing electrode 11 has to focus respective electron beams and converge the side beams Bs. However, in practice since the refractive index of the main electron lens is varied when the focusing voltage is adjusted to enhance the focusing characteristics, the shape of the equipotential lines between the electron beam passing holes 101, 103, 111 and 113 also varies. As a result, the focusing characteristics are varied so that the two requirements as above cannot be satisfied. In addition, since the convergence rate must be varied depending upon the size of the color cathode-ray tube, there occurs a problem in that the eccentricity .DELTA.S must be adjusted properly in correspondence with the size of the color cathode-ray tube, and also a further problem occurs in that the number of parts of the second accelerating and focusing electrode 11 is large so that the workability for assembling the electron gun becomes lower.
Furthermore, in the color cathode-ray tube which adopts a circular symmetrical lens system, although a thin and round beam spot can be obtained at the center of the fluorescent screen by a strong quadruple magnetic field within a color cathode-ray tube having a deflection yoke of non-uniform magnetic field for self-convergence, a flare with a low electronic density is formed at the circumferentical portion of the beam spot so that the focusing characteristics are deteriorated and thus the resolution of the color cathode-ray tube becomes lower.
Self-convergence is a method for directing three electron beams to focus on a point by deflection of electron beams even at the circumferential portion of the screen of a color cathode-ray tube. That is, the magnetic forces applied to three electron beams form non-uniform magnetic fields by means of the deflection yoke positioned just before the electron gun 2 as shown in FIG. 1. By such an arrangement, although the self-convergence characteristics may be obtained, it is inevitable that the focusing characteristics of electron beams become deteriorated.
Considering the problems mentioned above, an electron gun with a convergence structure as shown in FIGS. 5A and 5B has been proposed.
In such a type of electron gun, the second grid electrode 9 has longitudinal slots 94, 95 and 96 each of which has the same width as that of electron beam passing holes 91, 92 and 93. The slot 95 is positioned symmetrically with respect to the central electron beam passing hole 92 while other two slots 94 and 96 are eccentric with respect to the center of the side passing holes 91 and 93.
In FIG. 5A, the electron beam passing holes 101, 102 and 103 of the first accelerating and focusing electrode 10 and the electron beam passing holes 91, 92 and 93 of the second grid electrode 9 a conventional electron beam convergence structure are disposed in the same axial line, and the dimension of the slots 94, 95 and 96 in lengthwise is determined by the equation l1+l2=2l3 and l2&gt;l1.
According to this type of the conventional electron beam convergence structure, the equipotential lines V1, V2 . . . are formed asymmetrically on the slots 94 and 95 of the second grid electrode 9 which are disposed asymmetrically around the electron beam passing holes 91 and 93.
That is, at the outer position l1 where the length of the slot is short with respect to the center of the electron beam passing hole, the gradient of the equipotential lines is abrupt, while at the inner position l2 where the length of the slot is large the gradient thereof is gradual. The electron beams Bs which have been passed through the side electron beam passing holes 91 and 93 pass through the second grid electrode 9 are then converged into the central beam by refracting toward a center axis at a predetermined angle .theta..
The electron gun 2 having a convergence structure at the second grid electrode 9 achieves good convergence characteristics because the convergence structure between the first accelerating and focusing electrode 10 and the second accelerating and focusing electrode 11 compensates for the convergence deterioration caused by a variation of a convergence voltage. And, also since the slots 94, 95 and 96 strengthen the focusing operation in the breadthwise direction and deteriorates focusing in the longitudinal direction, the electron beams Bs, Bc and Bs passing through the passing holes 91, 92 and 93 are strongly focused in the breadthwise direction so that a longitudinally extended electron beam is formed and then neutralized with an inverse quadruple magnetic field while passing through the main electron lens and the asymmetric magnetic field for self-convergence, thereby forming a beam spot of low density and low flare on the fluorescent screen, resulting in increased resolution of the color cathode-ray tube.
However, the above-mentioned second grid electrode 9 as shown in FIGS. 5A and 5B has a disadvantage in the manufacturing thereof.
That is, since the slots 94 and 96 are formed eccentrically with respect to the passing holes 91 and 93, the manufacturing of a mold for the eccentric slots is difficult and also it is very difficult to adjust the amount of eccentricity precisely in the pressing work. Furthermore, since the dimensions of the slots must be changed in accordance with the size of the color cathode-ray tube, additional molding work is required in each case.